The present invention relates to an integrated optical module in which an optical waveguide element is mounted on a platform (base) having an optical waveguide circuit.
As the demand for a broadband multimedia communication service such as the Internet increases explosively, development of a larger-capacity, higher-performance optical fiber communication system is sought for. The number of optical communication modules used in such a large-scale system increases more and more as the system size increases. Not only the number of optical communication modules but also the size thereof increases the cost and mounting load of the optical communication modules occupying the whole system to a non-ignorable degree. Therefore, size reduction, function integration, and cost reduction of the optical communication modules themselves are very critical problems.
As a countermeasure that solves the above problems, demand has arisen for a photonic integrated circuit (PIC) in which a plurality of optical elements are monolithically integrated on one substrate to realize a specific function, and optical and electrical integrated modules in which peripheral electronic circuit elements for driving an optical element are integrated. In particular, a hybrid optical integrated module in which an optical waveguide element is flip-chip mounted on a platform having an optical waveguide circuit is promising in terms of productivity and the like as an optical integration technique which is the most practical technique.
In an optical module manufactured by using such a hybrid optical integration technique, the signal input/output end faces of an optical waveguide circuit and optical waveguide element formed on one platform oppose each other through a narrow air gap. In this air gap, mode field mismatching occurs between the two optical waveguides. A coupling efficiency caused by mode field mismatching is larger than that obtained with an aspherical lens or the like. This fact poses an issue in the hybrid optical integrated module when the module is to be applied to a high-end optical communication module aimed at a trunk optical fiber communication system, or an optical amplifier module.
To solve the problem of coupling loss in the hybrid optical integrated module described above, the following two methods are conventionally used. According to the first method, an air gap portion where the signal light incident/exit end face of an optical waveguide element and that of an optical waveguide circuit on the platform oppose is filled with a refractive index matching material such as a resin. According to the second method, in an optical waveguide element, for example, the sectional shape of a core layer in the vicinity of the signal light incident/exit end faces is changed along the longitudinal axis, so that the optical waveguide element has a spot size converting function.
Of the two methods, the first method using the refractive index matching material is performed by making a potting resin, which is originally used when hermetically sealing an optical waveguide element in order to increase the reliability, serve also to match the refractive index. This method has already been generally employed in a subscriber optical transmitting/receiving module. However, the refractive index of a medium that surrounds the optical waveguide element differs largely from that of air, and the effective end face reflectance of the optical waveguide element is inevitably largely adversely affected. Hence, when a refractive index matching material is to be used, the conditions (refractive index and thickness) of forming the end face coating of the optical waveguide element must be changed in advance in accordance with the refractive index of the refractive index matching material to fill.
As a result, the optical waveguide element performs originally designed operation only after filling the air gap with the refractive index matching material. The element characteristics of the optical waveguide element, before the optical waveguide element is flip-chip mounted on the optical waveguide platform cannot be evaluated or selected in advance which is a fundamental drawback. In particular, in hybrid optical integration of a semiconductor optical amplifier in which residual end face reflection must be minimized as much as possible, the end face coating does not serve as a low-reflecting film in a state before flip-chip mounting. In this case, a large ripple appears in the gain spectrum of the semiconductor optical amplifier, and the semiconductor optical amplifier does not operate properly as originally designed.
Also, in the semiconductor optical amplifier, an oblique optical waveguide structure or the like in which active layer stripes are inclined in the vicinity of the signal light incident/exit end face with respect to the direction of the normal to the end face is used to suppress residual end face reflection. In this case, however, as the air gap is filled with a refractive index matching material, the angle of refraction of the signal light changes in the air gap, and the optimal coupling position to the end face of an opposite optical waveguide circuit is undesirably shifted. Because of these reasons, in hybrid optical integration of a semiconductor optical amplifier, the countermeasure of improving the coupling efficiency by means of a refractive index matching material cannot be used.
The latter method employing the spot size converting function is also widely used as a countermeasure of improving the coupling efficiency of an integrated optical module. For example, a coupling efficiency of about xe2x88x921 dB to xe2x88x922 dB is obtained when a quartz-based optical waveguide is coupled to a single mode semiconductor laser, and a coupling efficiency of about xe2x88x924 dB to xe2x88x925 dB is obtained when it is coupled to a semiconductor optical amplifier. A coupling efficiency of this degree does not pose a serious problem in, e.g., a subscriber optical transmitting/receiving module that requires comparatively low performance requirements for an optical module. When, however, the hybrid optical integrated module is to be applied to a high-end optical module to be built into a trunk optical fiber communication system, a further improvement in coupling efficiency is desired. Currently, the application range of the hybrid optical integration technique is entirely limited to a low-cost optical module for a subscriber optical communication system.
It is, therefore, an object of the present invention to provide an integrated optical module in which the signal light coupling efficiency between an optical waveguide element and optical waveguide circuit with signal light incident/exit end faces opposing each other through an air gap is improved more effectively than in the prior art.
It is another object of the present invention to provide an integrated optical module in which size reduction, cost reduction, and improvement in mass productivity are realized.
In order to achieve the above object, according to the present invention, there is provided an integrated optical module comprising an optical waveguide circuit for guiding signal light, a base with a major surface where the optical waveguide circuit is formed, the base having a first end face where the signal light passing through the optical waveguide circuit emerges and becomes incident, an optical waveguide element mounted on the base and having a second end face opposing the first end face through a predetermined air gap, and a wave front compensating portion integrally formed with the base at a region of the first end face where the signal light becomes incident and emerges, to decrease a curvature of a wave front of the incident/exit signal light.